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Minerals 2018, 8(5), 181; https://doi.org/10.3390/min8050181

PGE–(REE–Ti)-Rich Micrometer-Sized Inclusions, Mineral Associations, Compositional Variations, and a Potential Lode Source of Platinum-Group Minerals in the Sisim Placer Zone, Eastern Sayans, Russia

1
Research Laboratory of Industrial and Ore Mineralogy, Cherepovets State University, 5 Lunacharsky Avenue, 162600 Cherepovets, Russia
2
Institute of Mining, Geology and Geotechnology, Siberian Federal University, 95 Avenue Prospekt im. gazety “Krasnoyarskiy Rabochiy”, 660025 Krasnoyarsk, Russia
3
Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, QC H3A 0E8, Canada
*
Author to whom correspondence should be addressed.
Received: 31 March 2018 / Revised: 18 April 2018 / Accepted: 19 April 2018 / Published: 27 April 2018
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Abstract

We report the results of a mineralogical investigation of placer samples from the upper reaches of the Sisim watershed, near Krasnoyarsk, in Eastern Sayans, Russia. The placer grains are predominantly Os–Ir–(Ru) alloys (80%) that host various inclusions (i.e., platinum-group elements (PGE)-rich monosulfide, PGE-rich pentlandite, Ni–Fe–(As)-rich laurite, etc.) and subordinate amounts of Pt–Fe alloys. Analytical data (wavelength- and energy-dispersive X-ray spectroscopy) are presented for all the alloy minerals and the suite of micrometer-sized inclusions that they contain, as well as associated grains of chromian spinel. The assemblage was likely derived from chromitite units of the Lysanskiy mafic–ultramafic complex, noted for its Ti–(V) mineralization. In the Os–Ir–(Ru) alloys, the ratio Ru/Ir is ≤1, Ir largely substitutes for Os, and compositional variations indicate the scheme [Ir + Ru] → 2Os. In contrast, in the laurite–erlichmanite series, Ir and Os are strongly and positively correlated, whereas Ir and Ru are negatively correlated; Ru and Os are inversely correlated. These compositions point to the scheme [Os2+ + 2Ir3+ + □] → 4Ru2+ or alternatively, to Os2+ + Ir2+ → 2Ru2+. We deduce a potential sequence of crystallization in the parental rock and address the effects of decreasing temperature and increasing fugacity of sulfur and arsenic on the assemblage. Inclusions of Ti-rich minerals in the alloy grains are consistent with the Lysanskiy setting; the complete spectrum of chromite–magnesiochromite compositions indicates that an important part of that complex was eroded. A localized fluid-dominated micro-environment produced the unique association of laurite with monazite-(Ce), again considered a reflection of the special attributes of the Lysanskiy complex. View Full-Text
Keywords: platinum-group elements; platinum-group minerals; PGE alloys; chromian spinel; schemes of substitution; Ti- and REE-rich inclusions; Sisim placer zone; Lysanskiy complex; Eastern Sayans; Russia platinum-group elements; platinum-group minerals; PGE alloys; chromian spinel; schemes of substitution; Ti- and REE-rich inclusions; Sisim placer zone; Lysanskiy complex; Eastern Sayans; Russia
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Barkov, A.Y.; Shvedov, G.I.; Martin, R.F. PGE–(REE–Ti)-Rich Micrometer-Sized Inclusions, Mineral Associations, Compositional Variations, and a Potential Lode Source of Platinum-Group Minerals in the Sisim Placer Zone, Eastern Sayans, Russia. Minerals 2018, 8, 181.

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